DecorView是如何添加到窗口的?

10.ActivityThread
11.WindowManager和WindowManagerImpl和WindowManagerGlobal
12.ViewRootImpl中通过addToDisplay添加DecorView并显示出来
13.view.assignParent设置DecorView的parent为ViewRootImpl，这也就是View在调用requestLayout方法时最终会调用ViewRootImpl的requestLayout方法的原因

从setContentView流程分析中我们可以看到DecorView是如何创建的，那么DecorView是如何添加到窗口中让我们可以看到的呢？

我们可以查看startAcitivty做了些什么？

Acitivity#startActivity(Intent intent)

1. 会直接调用startActivity(Intent intent, @Nullable Bundle options)

this.startActivity(intent, null);

2. 查看startActivity(Intent intent, @Nullable Bundle options)，最终会调用startActivityForResult(@RequiresPermission Intent intent, int requestCode,@Nullable Bundle options)方法

startActivityForResult(intent, -1);
startActivityForResult(intent, requestCode, null);

Activity#startActivityForResult

1. 调用Instrumentation的execStartActivity方法得到ActivityResult

Instrumentation.ActivityResult ar =
    mInstrumentation.execStartActivity(
        this, mMainThread.getApplicationThread(), mToken, this,
        intent, requestCode, options);

2. 如果ActivityResult不为null，则调用AcitivtyThread的sendActivityResult方法

if (ar != null) {
      mMainThread.sendActivityResult(
                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                    ar.getResultData());
}

Instrumentation#execStartActivity

1. 调用ActivityManagerProxy的startActivity方法，内部会调用远程服务的IBinder接口的transact方法

int result = ActivityManagerNative.getDefault()
    .startActivity(whoThread, who.getBasePackageName(), intent,
            intent.resolveTypeIfNeeded(who.getContentResolver()),
            token, target != null ? target.mEmbeddedID : null,
            requestCode, 0, null, options);

2. 调用Instrumentation的checkStartActivityResult方法检查Activity是否能被启动

checkStartActivityResult(result, intent);

AcitivtyThread#sendActivityResult

调用ApplicationThread的scheduleSendResult方法

mAppThread.scheduleSendResult(token, list);

ApplicationThread#scheduleSendResult

sendMessage(H.SEND_RESULT, res);

AcitivtyThread#sendMessage

1. 会直接调用sendMessage(int,Object,int,int,bolean)

sendMessage(what, obj, 0, 0, false);

2. 查看sendMessage(int,Object,int,int,bolean)，

Message msg = Message.obtain();
...
mH.sendMessage(msg);

H&sendMessage

H是Handler的子类，所以我们可以去查看H的handleMessage方法，前面startActivity的时候在ApplicationThread&scheduleSendResult时发送了一个msg.what = H.SEND_RESULT的Message：

public void handleMessage(Message msg) {
            switch (msg.what) {
                ...
                case SEND_RESULT:
                    handleSendResult((ResultData)msg.obj);
                    break;
                ...

AcitivtyThread#handleResumeActivity

...
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
...
a.mWindowAdded = true;
wm.addView(decor, l);

可以看到通过WindowManager最终将DecorView添加到相应的Window上去